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Hello. Thanks for the question.
It sounds like this is a nerve problem that you're seeing. If it started with one leg and now involves both legs, the most likely cause is a tumor pressing on his nerves and spinal cord. A spinal tumor is going to be the most likely, unfortunately.
There is not much you can do for this in a rat. Most end up being euthanized when we encounter this problem. If you really wanted to do everything, I would MRI and then try to remove the mass. You're talking about a few thousand dollars of diagnostics and surgery here. I let owners know it's an option, but I have yet to see any go for it- understandably.
I'm sorry I don't have any better news. Rats typically only live 2 years, and this is why- they tend to grow tumors quite readily.